Technical Field
The present disclosure relates to a filter. More particularly, the present disclosure relates to a filter for filtering infrared light.
Description of Related Art
Conventional optical systems constitute a set of lens elements and an image sensor, wherein the set of lens elements is disposed at an object side of the optical system and the image sensor is disposed at an image side of the optical system. Since the image sensor has high sensitivity to the infrared light, the infrared light thus may washout the color response in the visible spectrum and thus may distort the image color reproduction. Conventional infrared filter filters out the infrared light by applying alternate film layers of high refractive index (for example, TiO2, Ta2O5 or Nb2O5) and low refractive index materials (for example, SiO2 or MgF2).
In recent years, as the optical systems of the electronic products have gradually evolved toward compact size and wide viewing angle, the total track length of the optical systems has to be reduced and the chief ray angle also has to be large. Moreover, the conventional infrared filter tends to produce color shift in a peripheral region of an image as the chief ray angle becomes larger. Especially, when it is coated a multilayer with a high layer count, it tends to produce warpage due to uneven internal stress.